As the power of computers has rapidly increased, it has become obvious that a major limitation to the usefulness of computers involves the difficulty of entering data. As a result, numerous devices and schemes for data input and user interface have been developed. Many of these have utilized the idea of transferring positional information into digital format. Usually, a pointer or other device, sometimes called a marker, is used as an instrument for indicating the position of interest (although some devices have sought to detect the position of a pointed finger, or the like). A distinction is sometimes made between "mouse" type devices which control a cursor on a computer screen relative to marker movements and "digitizer" devices which impart positional data to a computer based upon marker position within a work surface area or the like. However, such a distinction primarily reflects differences in applications rather than in the devices themselves. Indeed, while a mouse type application generally does not require the high resolution or accuracy required of a digitizer type application, if a position locating devices is sufficiently accurate, it might be used in either type of application with appropriate supporting software.
Some such devices have employed any of several common intermediate steps. For instance, positional information has first been converted into variations of capacitance, voltage, inductance, frequency, signal phase, or the like. The intermediate analog is then converted, by conventional means, into a digital signal of a format suitable for digital computer input. This sort of positional "digitizer" has been universally accepted as a valuable input device type in many applications. The complication in this scheme is that it has proven to be very difficult to accurately interpret the position of a marker and to convert it into an intermediate analog. Among the many methods which have been used for this purpose have been devices which rely upon 14 variations in magnetic field strength and those which convert mechanical position into an electrical analog by means of movable coils or the like. This brief overview by no means purports to be a complete listing of the means which have been employed in the field, but each such means has suffered from at least one problem which has made it less than totally desirable. For instance, devices which rely upon variations in magnetic fields are overly susceptible to interference from stray fields, and electromechanical devices are prone to wear out or break, and are usually bulky and/or difficult to manipulate.
One solution that has been tried involves the use of ultrasonic waves. Briefly, since the speed of sound in a given environment can be determined with considerable accuracy, the time which it takes a sound wave to travel between two points is a good indicator of the distance between those points. Therefore, if a sound is emitted from a movable marker and the time taken for that sound to travel to two stationary receivers is determined, the relative position of the marker in two dimensions can be ascertained using simple trigonometric calculations. Use of three stationary references can allow three dimensional analysis. The same principles also apply if the transmitter/receiver functions are reversed, with the stationary references being the transmitters and the movable marker being the receiver. An example of this type of position determining apparatus is found in U.S. Pat. No. 4,758,691 issued to De Bruyne.
By way of example, De Bruyne and related prior art teach that ultrasonic pulses can be used to calculate distance by starting a high speed counter simultaneously with the emission of a pulse from a movable marker, and then stopping the counter when the pulse is received at one or more stationary positions. Further, since the speed of sound will vary somewhat with temperature and barometric pressure, in order for this measurement to be more accurate, some prior art methods have included means for adjusting measurements to correspond to the speed of sound under extant conditions. A common method has been to use a thermistor to determine ambient temperature, and to adjust a variable quantity accordingly in subsequent calculations.
While the sound wave pulse timing method has been widely employed in a variety of forms, it is recognized that certain qualities of sound waves and of the environments in which such devices are used impose appreciable limitations on the potential accuracy of such devices. The most basic such limitation involves the difficulty of precisely detecting a specific point in a received pulse which can be used as an accurate marker point in the signal. The solution taught by De Bruyne was to use a rapidly rising initial wave shape to create an acoustic shock wave. While this solution did offer an increase in attainable resolution it also created an undesirable audible tone at the frequency of repetition of the shock wave, and the resolution was still not as precise as is desired for many applications.
Yet another approach is exemplified by the teachings of U.S. Pat. No. 4,862,152 issued to Milner. Milner teaches a method for approximating the detection of a point in a series of individual cycles of a wave form within a pulse using a low pass filter to create an envelope wave shape of the pulse. A relative magnitude of this envelope is then used as a trigger point. Of course, since the accuracy of this method is entirely dependent upon the recognition of the appropriate trigger point magnitude, and since the magnitude of a received sound pulse falls off with the square of the distance over which it must travel, a means must be included for adjusting for the distance the sound pulse must travel. Milner used, for this purpose, a ramp voltage which increased in magnitude with time. The ramp voltage was then used to modify the gain of a receiving amplifier such that, the longer the time between emission and reception of the pulse, the greater the instantaneous gain of the amplifier. While this method has provided several significant advances in the field, it is still not as accurate as might be desired, since it does not identify with sufficient precision an exact point within a received pulse to accurately trigger the stopping of counters. Furthermore, this method is still quite prone to error caused by ambient noise interference, since it is entirely magnitude dependent.
All of the prior art methods for determining position using ultrasonic waves or pulses within the inventor's knowledge have not been capable of the fine resolution of which the present invention is capable, and/or have been less reliable than the inventive method, and/or have produced undesirable side effects such as unwanted audible noise.
No prior art positioning determining methods to the inventor's knowledge has successfully provided a means for determining position with sufficient high resolution to allow attitudinal measurements or to allow positional measurement to the degree of accuracy desired for many applications. All successful applications to date have been either too inaccurate or too unreliable for practical application.